This invention relates to laminates having at least one electrically conductive layer. More specifically, it relates to such laminates additionally having at least one static dissipative layer.
With the proliferation of electronic equipment, there has developed a rapidly growing market for packaging and other materials which protect sensitive electronic components from electrostatic charges and fields. Many electronic devices, including printed circuits and microchips, are extremely vulnerable to damage from static discharges of as little as 50 volts. Static discharges on the order of 10,000 volts are commonly encountered during the normal handling of these devices such as from friction, motion, separation of dissimilar materials, and induction. It is, therefore, necessary to protect the devices from electrostatic fields and charges during their manufacture, transportation and use. Packaging and other materials used in handling or manufacturing electrostatically sensitive devices desirably protect against rapid, damaging discharges and fields. In addition to packaging, such items as storage containers, housings for electronic devices, materials used in environments in which the sensitive devices are manufactured and the like advantageously also provide such protection.
Two types of protection are advantageously provided. First, the devices are shielded from external electrostatic or radio frequency fields. Shielding can be provided by conductive films or foils, for instance, of carbon-loaded polyethylene. Carbon-loaded polymers typically have surface resistivities between about 10.sup.3 and 10.sup.6 ohms per square. Second, induced or applied electrostatic charges are attenuated or dissipated away from the devices to avoid areas of high specific charge. Films having treatments involving quaternary or tertiary amines are often used to dissipate charges. Such films typically have surface resistivities between about 10.sup.10 and 10.sup.13 ohms per square, but a somewhat more conductive material is desirable. A material as conductive as carbon loaded polyethylene or a metal is, however, so conductive that it may conduct an electrostatic charge to an electronic device before the charge can be dissipated. Such problems encountered in dealing with static are further explained by D. C. Anderson in "ESD Control: To Prevent the Spark that Kills," Evaluation Engineering, Vol 23, No. 7, Jul. 1984, p. 20.
Conductive laminates potentially have a number of uses. Each application involves a number of desired characteristics. A packaging material, for instance, is desirably lightweight, strong, and easily fabricated into a configuration which is adapted to the size and shape of an electronic device. While polymers, particularly polyolefins, possess the foregoing characteristics, they are not generally static dissipative or conductive without surface treatment, incorporation of a blooming additive or filling with carbon black, graphite or metallic particles. To achieve conductivity as required for shielding, a polymer must have a relatively high loading of conductive filler therein which loading is believed to provide sufficient conductive filler to achieve contact between conductive particles. In the case of carbon black, 25-40 percent by weight carbon black based on total weight of carbon black and polymer is often required. Such a highly carbon black filled polymer is referred to as carbon-loaded. Such concentrations of carbon black or other fillers typically render the polymers mechanically weak, and thus degrade some of the properties that render polymers suitable for packaging. Furthermore, loaded films are known to contaminate electronic devices by sloughing filler particles.
Layers of polymers filled with sloughable materials have been laminated between polyethylene layers to render the filled layers more manageable in a packaging process and reduce contamination by sloughing. The laminates of U.S. Pat. Nos. 4,554,210 and 4,590,741 exemplify laminates having filled layers. Such laminates, however, have relatively high surface resistivities generally characteristic of the outer layers of the laminate because the outer layers electrically insulate the inner layer. Surface resistivity can be lowered, for instance, by use of a conductive plasticizer or surface treatment on one or both outer layers as exemplified, for instance, by the laminate of U.S. Pat. No. 4,363,071.
When an amine, humectant or surfactant compound is incorporated into a polymer or used as a static dissipative surface treatment thereon, such surface treatments exude to the surface of the polymer, where they absorb atmospheric moisture to form an electrolyte microlayer. The microlayer is generally sufficiently conductive to render the polymer static dissipative. At least four major problems are encountered with static dissipative surface treatments. Since the static dissipative agents are on the surface, they are subject to being removed during handling. In this manner, the static dissipative effect is reduced or destroyed until more of the amine, humectant or surfactant can migrate to the surface. Further, since the static dissipative agent is being continually removed, the polymer will eventually lose its static dissipative properties. In addition, these static dissipative agents depend on a humid environment for effective operation. Thus, their static dissipative behavior will vary according to the local relative humidity, and will be minimal in arid environments. Finally, these static dissipative agents are sometimes corrosive or are potential contaminants.
Accordingly, it would be desirable to provide a system of polymers which has conductive and static dissipative properties which are not significantly dependent on local humidity, are stable over time, and which has physical properties which permit it to be used for a variety of packaging and other shielding and static control and conductive applications.